A conventional vertical-to-surface optical semiconductor device is described on pages 799 to 801 of "Appl. Phys. Lett. 60(7), Feb. 17, 1992", in which the conventional vertical-to-surface optical semiconductor device operates as a vertical cavity laser.
The conventional vertical-to-surface optical semiconductor device comprises, especially, a first mirror layer, an active layer, and a second mirror layer successively grown on a substrate, wherein the layers are mesa-etched to be square at the horizontal cross-section and to confine light in the transverse direction, such that a cavity is formed between the first and second mirror layers.
The vertical-to-surface optical semiconductor device also operates as a photodetector. For this function, first and second vertical-to-surface optical semiconductor devices may be optically coupled, such that the second vertical-to-surface optical semiconductor device receives an optical signal emitted from the first one. Thus, an apparatus for coupling optical signals is structured to exchange optical information between the first and second vertical-to-surface optical semiconductor devices.
In the conventional vertical-to-surface optical semiconductor device, however, there is a disadvantage in that a lasing wavelength shifts in a direction of longer wavelengths due to the heat generated therein during lasing operation. Consequently, the detection precision is lowered in the optical signal coupling apparatus comprising the first and second vertical-to-surface optical semiconductor devices, because the light receiving device has no shift in the oscillation wavelength, while the light emitting device has the shift of the lasing wavelength, as described above.